Rotary hearth calciners are well known in the art and have been employed with considerable success for the processing of carbonaecous materials therein with a considerable savings in the amount of fuel required to process the said materials. Such calciners have been employed in the treatment of volatile containing substances such as, for example, non-caking coal, anthracite coal, wood products, green petroleum coke, pellets or briquettes containing controlled percentages of bituminous caking coal and other carbonaceous materials or inerts either with or without a bituminous binder. Such calciners have also been employed in the calcining of dolomite, limestone, and cement rock, the reclaiming of calcium oxide from carbonate sludge, the decomposition of carbonates, sulphates and chlorides, the reactivation of activated carbon, and/or the like. Also, such calciners have been employed in the simultaneous production of metallized ores and coke from oxide pellets or other feeds rich in metal oxides which are charged into the calciner along with a carbonaceous material, such as coal or the like.
The known calciners comprise a substantially air-tight enclosure having a roof, sidewalls and an imperforate rotating or traveling hearth upon which the materials to be processed are deposited. The materials to be processed contain volatiles which evolve therefrom when exposed to heat. At the start of a calcining operation for the production of coke, or for the simultaneous production of metallized ores and coke, the furnace enclosure is brought to operating temperatures, that is, the temperature within the furnace is such that some of the volatiles in the carbonaceous materials which have been deposited on the hearth are evolved therefrom and caused to travel upwards within the furnace to a position closely adjacent the roof thereof where air and/or an air-oxygen mixture is admitted into the furnace and to there comingle with the evolved volatiles to produce an oxidizing atmosphere in that portion of the furnace while maintaining a reducing atmosphere about the materials undergoing treatment on the hearth. The burning of the volatiles in that portion of the furnace will heat the roof and sidewalls thereof so that the heat radiated from the roof and sidewalls and also from the oxidizing volatiles in the upper portion of the furnace will result in a continued removal of volatiles from the materials undergoing treatment on the hearth. The continued removal of volatiles from the materials undergoing treatment, as aforesaid, and the continued supply of air to the upper portion of the furnace may, in some instances, supply all of the heat to properly process the materials undergoing treatment on the hearth and to permit for the continued operation of the furnace on a autogenetic basis, whether the calciner is employed to produce a coke from a carbonaceous volatile containing material such as set forth previously or whether the calciner is employed to simultaneously produce a metallized ore and coke from a combined charge of metal oxide pellets, or the like, and a carbonaceous material. In instances where the carbonaceous materials do not release sufficient volatiles to permit for the continued operation of the calciner on an autogenetic basis, outside heat may be supplied in any suitable manner, for example in the upper portion of the enclosure by burners mounted either in the roof or sidewalls of the enclosure to thereby insure the proper processing of the materials which have been deposited on the hearth.
The aforesaid known calciners are designed to operate on a continuous basis with materials being continuously introduced onto the hearth and following the processing thereof, the materials are then directed to a suitable outlet. In the case of a rotating hearth having a central outlet for the processed materials, roof mounted rabbles may be employed for progressively advancing the materials to the central outlet. In instances where such rabbles are not used, suitable scraper means may be employed to extend over the traveling hearth to direct the processed materials outwardly of the hearth to a suitable receiver means located in close proximity to the calciner.
Previously known calciner structures have not been concerned with a problem which has resulted in a poorer yield of processed materials than that attained by employing the method and apparatus set forth with particularity in the ensuing description of the invention.